Zinc oxide (ZnO) power varistors are nonlinear, voltage-dependent resistor bodies that comprise ceramic sintered bodies based on zinc oxides as the resistance element. For varistors the electrical resistance decreases strongly with rising voltage above a response voltage. Due to this electrical behavior, varistors are used to protect electrical systems and equipment from overvoltages and voltage peaks. The varistor in this case is connected in parallel to the electrical system to be protected, and by virtue of its current-voltage characteristic, limits the maximum voltage appearing at the electrical system. Electrodes for electrical contacting of the varistors are applied to both end faces of the cylindrical main body of the varistors.
Overvoltages and voltage peaks can be subdivided on a time axis roughly into lightning strike overvoltage (time range: microseconds), switching overvoltages (time range: milliseconds) and temporary overvoltages (time range: seconds). Overvoltages in the microsecond range, in particular, can reach very high voltage peaks. Not only do these very fast and very high voltage peaks stress the zinc oxide ceramic of the varistor strongly, a breakdown also occurs without suitable countermeasures on the outer side or surface of the varistor.
A zinc oxide varistor in which the generated surface of the ceramic base body is coated with a high-resistance layer is known from U.S. Pat. No. 5,294,909. The crystallized glass composition for wetting the ceramic base body comprises lead oxide (PbO) as its main component and is enriched with the components ZnO, B2O3, SiO2, MoO3, WO3, TiO2 and NiO to promote the crystallinity and the insulating property of the layer. The addition of larger amounts of PbO to the insulating layer raises its coefficient of thermal expansion, the addition of larger amounts of ZnO enabling the crystallization of the glass composition of the layer. The addition of larger amounts of B2O3, on the other hand, leads to a reduction of the crystallization of the layer, particularly if the weight proportion of B2O3 exceeds 15%. The elevation of the SiO2 also leads to the reduction of crystallization, with simultaneous elevation of the coefficient of thermal expansion.
Arresters consisting of varistors are subject in use over long periods (service life≧30 years) to environmental influences such as moisture and chemical contaminants. There is the danger that these environmental influences may lead to a reduction of the varistor's ZnO ceramic and change the current-voltage characteristic. The function of protection from environmental influences is taken on here by the protective coating.